Really confusing math problem?

Question

There is this really hard homework problem that involves weighted averages. The question is:

A pinapple drink contains 15% pineapple juice. How much pure pineapple juice should be added to 8 quarts of the drink to obtain a mixture containing 50% pineapple juice?

Please tell me the answer and tell me how you solved it!!!

I do not understand how rec got from, x + 1.2 = .5x + 4, to, .5x = 2.8

Never mind my bad.

Answer 1

.15(8)+ 1(x) = .50(8 +x)

Answer 2

let x be the amount of pure pineapple juice added

the total amount of drink is x + 8, in which 50% are pure pineapple juice

.15(8) = 1.2 is the amount of pure pineapple juice in the 8 quarts drink

x + .15(8) = .5(x + 8)
x + 1.2 = .5x + 4
.5x = 2.8
x = 5.6 quarts <== answer


what i did was i put the x's on one side and the non-x's on the other side

x + 1.2 = .5x + 4

subtract .5x for both sides
.5x + 1.2 = 4

subtract 1.2 for both sides
.5x = 2.8

divide .5 for both sides
x = 5.6

Answer 3

well thats a very very very very very very ez question... first u take 15% and subtract 9% and then add 80% and u come up with 27%...how did u not know that?